Liquid crystals in flat panel displays have been used for many years, such as those used in watch faces or half page size displays for lap-top computers and the like.
Chiral nematic liquid crystal (or Cholesteric Liquid Crystal Material) material can be energized by application of a voltage to exhibit different optical states. Four representative states (textures) for the chiral nematic material are homeotropic, planar, transient planar, and focal conic. When in the homeotropic state, the liquid crystal material is transparent to normally incident light impinging upon the liquid crystal material. When in the focal conic state, the liquid crystal material weakly scatters the light, although if the path length is short enough material in this the state can appear transmissive, or black particularly when the back substrate is painted black. When in the planar state, the liquid crystal material reflects a pre-determined bandwidth of light. The final display state of each pixel of the liquid crystal display is typically selected to be in either the focal conic or planar state. The liquid crystal in the planar state reflects the light impinging upon the display to appear “light”, and the liquid crystal in the focal conic state will appear transparent (black with a black background) to provide sufficient contrast with the planar pixels.
Bistable chiral nematic displays made up of reflective chiral nematic materials do not require continuous updating or refreshing. The electronics update the display when data or information is changed. However, if the display information does not change, the display does not need to be updated. Early bistable cholesteric liquid crystal displays suffered from long update times due to the long transition times between the planar and homeotropic states. This limited the applications of chiral nematic liquid crystal displays to those that could tolerate slow updates.
As cholesteric liquid crystal display technology developed, drive schemes such as that described in the technical paper by X. Y. Huang entitled “Dynamic Drive for Bistable Reflective Choleseric displays: A Rapid Addressing Scheme” that was published in the SID 1995 Technical Digest, and disclosed in U.S. Pat. No. 6,268,840 to Huang, incorporated herein by reference in its entirety, substantially reduced the update time for liquid crystal displays. The '840 patent discloses a “dynamic drive” method of implementing a dynamic drive scheme by applying unipolar row and column voltages to achieve a bipolar resultant pixel voltage using relatively simple drive circuitry while maintaining the update time at less than 1 millisecond per row of pixels. The '840 patent controlled the Root Mean Square (RMS) value of the voltage applied to each pixel to achieve the desired pixel state quickly by taking advantage of rapid transition of the cholesteric liquid crystal material from the homeotropic to the transient planar state. Sequences of discrete voltage levels are applied to the pixel to achieve the desired RMS values.
The dynamic drive scheme disclosed in the '840 patent is based on various properties of the liquid crystal material. While not wanting to be bound by theory, some of those properties will be summarized herein. Transition between states occurs at different rates. For example, the transition between the homeotropic state and transient planar state is relatively fast (on the order of 1 millisecond at room temperature). The dynamic drive scheme makes use of this rapid transition when the liquid crystal material is holding in the homeotropic state and the electric field is reduced (by changing the pixel voltage) below a critical level known as EHP*. The transition from the homeotropic to the focal conic state is slower (on the order of 10-100 milliseconds at room temperature). This transition takes place when the electric field is reduced to a level EHF that is generally higher than EHP*.
According to the '840 patent, the pixel voltage is applied in four phases, preparation, selection, evolution, and non-select as can be seen in FIG. 1. The top waveform in FIG. 1 depicts a pixel being addressed to the focal conic state and the bottom waveform depicts a pixel being addressed to the planar state. In the preparation phase, a preparation voltage of about 50 volts RMS is applied to the pixel to transform the liquid crystal material in the pixel to the homeotropic state. With the liquid crystal material in the homeotropic state, if the electric field is above EHF (as in the selection voltage in the bottom waveform shown in FIG. 1), the liquid crystal will stay in the homeotropic state because the homeotropic state is metastable or stable. If the electric field is below EHF and above EHP* (as in the selection voltage in the top waveform shown in FIG. 1), the liquid crystal will transform into the focal conic state. When the electric field is reduced further below EHP*, the transition from homeotropic to the transient planar state also becomes possible. Competition between the two transitions (homeotropic to focal conic and homeotropic to transient planar) takes place and the faster transition (homeotropic to transient planar) will eventually dominate.
During the short selection phase of the dynamic drive, when the selection voltage is low (see top waveform of FIG. 1), the liquid crystal transforms into the transient planar state. When the selection voltage is high (bottom waveform of FIG. 1), the liquid crystal starts to transform to the focal conic state. However, the homeotropic to focal conic transition is very slow and at the end of the short selection phase, the liquid crystal is still mainly in the homeotropic state. To implement gray scale using the dynamic drive, the RMS of the selection voltage is varied to control the mixing ratio of the planar and focal conic states within a pixel to achieve a desired shade of gray. “Grayscale of Bistable Reflective Cholesteric Displays” by Xiao-Yang Huang et al. published in SID98 Technical Digest describes one way of using the dynamic drive and its derivative schemes to obtain gray scale.
During the evolution phase, a relatively high voltage (about 31 volts for typical display materials) is applied to the pixel so that the liquid crystal material is either maintained in a homeotropic configuration or evolves into a focal conic state. After the evolution voltage is removed, the pixel transforms to its final state, either focal conic or planar, depending upon the voltage applied during the selection phase. A relatively low (about 5V) RMS non-select voltage is maintained at the pixel until the end of the display update or the next preparation phase begins.
To achieve the drive scheme of the '840 patent, a sequence of waveforms are utilized to provide the desired resultant RMS pixel voltage for each phase in the drive scheme. These waveforms are selected to allow the creation of the necessary RMS voltages while preventing any net DC voltage. Each waveform is defined in four distinct drive phase sequences T1, T2, T3, and T4. A typical implementation of the dynamic drive scheme is shown as a pixel waveform for the planar or “on” pixel case in FIG. 2. This example implements a selection period of states T1, T2, T3, and T4. In order to reduce power, the time spent in each state can be increased and a two selection state waveform can be utilized as shown in FIG. 3. In the FIG. 3 case, the following row addressed will see a waveform with T3 and T4 selection states. Both the sequence shown in FIG. 2 and that shown in FIG. 3 will result in the same RMS waveform being applied to the pixel.
While the liquid crystal display described in the '840 patent performs well in terms of update speed and display quality with cost effective components, image degradation has been detected as well as a decrease in image update speed when certain sequences of drive states are applied to the pixels. In particular, the inventor of the present waveform sequencing method and apparatus has observed that adjacent rows of pixels that should have the same appearance alternate between “light” and “dark” in appearance. This “banding” effect can be most easily detected in the case of repeated focal conic (“dark”) pixels including gray scale pixels.